Control system for once-through boilers

ABSTRACT

The control system controls the fuel flow rate, during starting of a once-through boiler, and includes detectors measuring the feed water temperature at the outlet of a water-cooled furnace wall and the gas temperature at the outlet of the furnace. The system includes further detectors detecting abnormal conditions with respect to the feed water temperature and the gas temperature, and setting means for setting the final desired temperature of the feed water during starting and the temperature of the feed water during full operation. The control is effected by pulses from a pulse generating circuit operable to generate a starting pulse, control pulses having a predetermined duration and predetermined intervals, and holding pulses having a phase and timing equal to those of the control pulses. A setting point generating circuit is selectively switched into a follow-up mode in which there is no starting pulse and a setting point equal to the actual temperature of the feed water at the outlet of the water-cooled furnace wall is generated, a temperature rise mode in which the start pulse is generated and the holding pulses are generated, and the setting temperature is varied during predetermined time intervals in accordance with a generated temperature change rate, and a holding mode in which the setting temperature is held at a constant level.

United States Patent [191 Fujii et al.

[451 Aug. 13, 1974 CONTROL SYSTEM FOR ONCE-THROUGH BOILERS Inventors: Masaru Fujii; Syuji Oyagi; Tutomu Kamei; lnosuke Mori, all of Nagasaki, Japan Mitsubishi Jukogyo Kabushiki Kaisha, Tokyo, Japan Filed: Aug. 2, 1973 Appl. No.: 384,839

Assignee:

Foreign Application Priority Data Aug. 4, 1972 Japan 47-78207 US. Cl 122/406 ST, 122/448 S, 122/451 S int. Cl. F22lb 35/14 Field of Search 122/406 S, 406 ST, 448 S,

Primary E.raminer-Kenneth W. Sprague Attorney, Agent, or FirmMcGlew and Tuttle [57] ABSTRACT The control system controls the fuel flow rate, during starting of a once-through boiler, and includes detectors measuring the feed water temperature at the outlet of a water-cooled furnace wall and the gas temperature at the outlet of the furnace. The system includes further detectors detecting abnormal conditions with respect to the feed water temperature and the gas temperature, and setting means for setting the final desired temperature of the feed water during starting and the temperature of the feed water during full operation. The control is effected by pulses from a pulse generating circuit operable to generate a starting pulse, control pulses having a predetermined duration and predetermined intervals, and holding pulses having a phase and timing equal to those of the control pulses. A setting point generating circuit is selectively switched into a follow-up mode in which there is no starting pulse and a setting point equal to the actual temperature of the feed water at the outlet of the water-cooled furnace wall is generated, a temperature rise mode in which the start pulse is generated and the holding pulses are generated, and the setting temperature is varied during predetermined time intervals in accordance with a generated temperature change rate, and a holding mode in which the setting temperature is held at a constant level.

8 Claims, 7 Drawing Figures FLOW RATE Fw w FLOW SETTER I RATE com 2' (D FIRST g 4- OPERATION MEANS c Fl! 6 TEMP RISE iH+ 8 59 F *Fflil FUEJFLDW Isl} RATE SETTER 7 fi/g fl fi' Wq- TRA E CONT BOILER IO -H0L0|-Z ZLMP 31 MEANS MEANS pa 5 24\'AR|THMETIC 23 (3 s Fg NI Tg-TgL COMPARATOR 6 q LIMIT SAMP PULSE gwigs GEN I5; I

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SHEET 3 0? 7V SAMPLING PULSE TEMPERATURE FUEL FLOW RATE PATENTED Aum 31974 -3'.8-2 8'.737 saw u or 7 FIG}. 4

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TIME (min) PATENTEU we] 3 I974 SHEET 5 0F 7 FIG. 5

SAMPLING PULSE 0 AT 2m 3AT 7AT BAT Tw(O) TIME MKDPQKMQEMP CONTROL SYSTEM FOR ONCE-THROUGH BOILERS FIELD AND BACKGROUND OF THE INVENTION This invention is directed to an automatic control system for controlling the flow rate of fuel when a once-through boiler is started.

When a supercritical-pressure once-through boiler is started, the temperature rise rate of the feed water at the outlet of a water-cooled furnace wall is limited to less than, for example, 220C/h, and the temperature of the gases at the outlet of the furnace is limited to less than, for example, 538C in order to protect the superheater and re-heater as well as to prevent destructive stresses and thermal shocks from being causes in welded joints.

The mode of operation and the control of a oncethrough boiler will be described with reference to FIG. I. Feed water is fed from a feed water pump 01 through a feed water heater 02, and economizer 03 and a mixing ball 04 to a circulation pump 05 which, in turn, forces the feed water into a water-cooled furnace wall 06. When the boiler is started and is in the initial stage, the heated water is supplied to a water and steam separator 08 through an extraction valve 07, because both a boiler throttle bypass valve 012 and a boiler throttle valve 013 are closed. Steam from separator 08 is supplied through a boiler steam admission valve 09 to a superheater 010. A part of the water from furnace wall 06 is returned through a recirculation valve 011 to mixing ball 04, in which it is mixed with water supplied from economizer 03. When the boiler is started, the flow rate of the feed water is maintained constant, and, in general, the minimum permissible flow rate, which is 5 percent of a rated flow rate in a forced oncethrough boiler and 25 to 30 percent of the flow rate in an ordinary once-through boiler, is maintained.

The water pressure at the outlet of furnace wall 06 is controlled to a predetermined value by an extraction or relief valve 07. Thus, the temperature rise of the water, when the boiler is started, is controlled at a constant flow rate of feed water. The temperature rise rate of the water at the outlet of the furnace wall, and the temperature of gases at the outlet of the furnace, are maintained within the above limits, and a fuel flow rate is so controlled that the temperature rise rate of the water at the outlet of the furnace wall is maintained substantially constant. Under these conditions, the boiler is warmed up.

When the steam conditions at the outlet of the boiler coincide with the turbine metal temperature, steam is supplied to a turbine 014 whose speed is gradually increased. When the turbine attains a rated speed, the generator which is coupled to the turbine is connected to power grid line. In this case, temperature control is continued by controlling the fuel flow rate until the water at the outlet of the water-cooled furnace wall reaches a predetermined temperature. When the quality of feed water is not satisfactory when the boiler is being warmed up, control of the fuel flow rate is also required in order to maintain the temperature of the water at the outlet of the furnace wall at a constant value.

Hitherto, in order to attain these two controls, operators must control the temperature of water at the outlet of the watencooled furnace wall as well as the temperature of gases at the outlet of the furnace, using the reading of a recorder or indicator. The operators are required to maintain these two controlled values within the above limits, so that the firing rate may be controlled within a short time. It follows therefore that the water at the outlet of the water-cooled furnace wall must have its temperature increased uniformly at a maximum rate within the above limit. However, the operators can control only based upon their judgment of the physical properties of the circulating water, which change in response to the temperature rise, in response to the reading of the temperature recorder. Alternatively, the operators effect a control based upon the temperature rise rate as read from a temperature recorder, by using a scale. Since monitoring and controlling of the fuel flow rate and temperature rise will generally last for 3 hours, highly proficient operators are required and even then, the accuracy of control by manual operation is low with personal errors.

SUMMARY OF THE INVENTION In accordance with the present invention, the firing rate is controlled automatically in response to certain important controlled values when a supercritical oncethrough boiler is started, so that the burden on the operators may be relieved and manual operation may be eliminated. The temperature rise rate of the water at the outlet of the water-cooled furnace wall, and the temperature of gases at the outlet of the funace, are precisely controlled within the required limit so that the temperature of the circulating water may be increased at a uniform rate and the boiler may be started within a short time in an economical manner.

The control system in accordance with the present invention, and capable of attaining the above and other objects, has the following features and advantages:

1. The temperature rise of the water at the outlet of water-cooled furnace wall is effected at a constant rate, in other words, the temperature change rate is constant.

2. The control system is not based upon the conventional principles in which control is effected in response to the result of direct differentiation or incomplete differentation of a controlled variable, such as the temperature of feed water. Such a method has many defects from the standpoint of accuracy, stability, complexity, etc. Furthermore, the control system is not based upon response to an average change rate obtained from an integrater or by utilizing the information storage properties of a magnetic tape. The temperature rise rate is controlled by means of simple, arithmetic operation units for individual systems, such as a basic fuel flow rate determination system, a feed forward control system and a feedback control system.

3. Control is possible with any desired temperature change (rise) rate of the feed water.

4. A holding control for maintaining a constant temperature is possible.

5. The temperature of the feed water at the outlet of the water-cooled furnace wall can be automatically increased at a rate within the limits in a safeguarded manner and without requiring manual operation.

6. The control system may be used at any point of the temperature raising operation, so that a smooth change-over to automatic control may be assured.

7. The control system is of the intermittent type and is adapted to be used as a direct control system with an electronic digital computer, and to be applied to the starting system with ordinary control devices in combination with digital control devices.

In general, the temperature of gases at the outlet of the furnace changes within a minute in response to a change in the fuel flow rate or firing rate. However, the temperature of the water at the outlet of the watercooled furnace takes from minutes to change after a change in the firing rate. That is, the response of the temperature change of gases at the outlet of the furnace is considerably different from the response of the temperature change of the water at the outlet of the water-cooled furnace wall, and control of a fuel flow rate may be made only when the controlled variables are in excess of a predetermined value. For this reason, in accordance with the present invention, a control circuit for controlling the temperature of gases at the furnace outlet, that is, a gas temperature circuit and a control circuit for controlling the temperature rise rate of water at the outlet of the water-cooled furnace wall, that is, a liquid temperature rise rate circuit, are provided independently of each other and combined in operation.

An object of the invention is to provide an improved control system for once-through boilers.

Another object of the invention is to provide such a control system which is automatic in operation.

A further object of the invention is to provide such a control system with which a once-through boiler may be started in a short time in an economical manner.

For an understanding of the principles of the invention, reference is made to the following description of a typical embodiment thereof as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the Drawings:

FIG. 1 is a block diagram of a forced once-through boiler;

FIG. 2 is a block diagram of a control system for controlling the firing rate or fuel flow rate when a oncethrough boiler is started;

FIGS. 3 and 4 are graphs explaining a control circuit for controlling the automatic temperature rise, explaining the control circuit for controlling the automatic temperature rise, at a constant rate, in water at the outlet of a water-cooled furnace wall;

FIG. 5 is a graph explaining the mode of operation of a circuit for automatically controlling the temperature of gases at the outlet of the furnace;

FIG. 6 is a detailed block diagram of a control system embodying the present invention; and

FIG. 7 graphically illustrates the data obtained from experiments conducted with a test plant utilizing the control system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Reference will be made first to FIG. 2 which illustrates the underlying principle of the present invention, and a liquid temperature rise circuit and a gas temperature circuit illustrated in FIG. 2 will now be described.

In the liquid temperature rise rate circuit, the symbols have the following meanings:

1 time AT period of intermittent control Ffli) fuel flow rate Ffc(i) fuel flow rate in the i-th step fuel flow rate correction in the i-th step F fb basic fuel flow rate AFfli) change in fuel flow rate from the (i 1) step to the i-th step Ts liquid temperature set Tw liquid temperature (at the outlet of the watercooled furnace wall) Tw(i) liquid temperature at the i-th step Tw(O) initial liquid temperature (liquid temperature when the control is changed over to automatic mode by using the present invention).

The change in fuel flow rate required to increase the temperature of water at the outlet of the water-cooled furnace wall, at a predetermined liquid temperature rise rate, is given by:

where C constant. In order to introduce the intermittent control, Eq. 1 is rewritten in the form of where K constant less than a limit.

When the period of the intermittent control AT is constant, ATs(i) becomes a constant H which is dependent upon AT and a temperature rise rate setting. That ATs(i) H Therefore c 2 1w ATw(i)} 0n the other hand,

ATw(i) Tw(i) Tw(i 1) Substituting this in Eq. 4, results in f U) If (i) l Therefore the fuel flow rate in the j-th step is given by From Equation 5, it will be seen that the flow rate F f(;') in the j-th step consists of the first term l of the basic fuel flow rate F jb, which is dependent upon a feed water flow rate, the second term (2) or CjH, or the fuel flow rate to be added in response to an expected increase (the feed forward control), and the third term (3) of the fuel flow rate correction (feedback control) ClTwLj) Tw(O)] obtained by detecting and feeding back Tw(j).

The liquid temperature rise rate circuit based upon the above principle is illustrated, in FIG. 2, above the dash..dash chain line, and includes the boiler l and the feed water flow rate setter 2 by means of which the feed water flow rate required when the boiler is started is set and a representative signal is generated. A feed water flow rate controller 3 is connected between center 2 and boiler l and controls the flow rate of feed water to be supplied to boiler I in response to the output signal from the feed water flow rate setter 2. A first proportional arithmetic operation means 4 has an input terminal connected to the output terminal of feed water setter 2, and has its output connected to a first input terminal of a first adder 5 to apply the plus or positive input signal to the adder.

A fuel flow rate controller 6 is connected to first adder 5 and, in response to the output of first adder 5,

fuel flow rate controller 6 controls the flow rate of fuel to be supplied to a starting burner in boiler 1. The circuit further includes a temperature rise rate setter 7, in which a temperature rise rate for the water at the outlet of water-cooled furnace wall, is set when the boiler is started and, in response to a sampling pulse which is generated for every control period or cycle, the output is increased by H. The output terminal of temperature rise rate controller 7 is connected to a first input terminal of a second adder 8 to apply a positive input to the second adder. A second arithmetic operation means 9 has its input terminal connected to the output terminal of second adder 8, and the output terminal of means 9 is connected to the second input terminal of first adder to apply a positive input thereto.

An initial liquid temperature setting means ill) is provided, and the temperature of water at the outlet of the water-cooled furnace wall of boiler I, when the automatic control system of the present invention is actuated, is set thereby at an initial value. A temperature detector 11 continuously detects the temperature of water at the outlet of water-cooled furnace wall and its output is applied to a first input terminal of a third adder 12 as a positive input, whereas the output of the initial temperature setting means is applied to a sec- 0nd input terminal of third adder 12 as a minus or negative input. A sampling means 13 has one of its input terminals connected to the output terminal of third adder l2, and a holding means 14 has its input terminal connected to the output terminal of sampling means 13 and its output terminal is connected to the second input terminal of second adder 8 in order to apply thereto a minus or negative input.

A relay means 15 included in the circuit has one contact connected to the sampler exciting terminal of sampling means 13 and to the sampling pulse input terminal of temperature rise rate setter 7. A sampling pulse generator 16 has its output terminal connected to the other contact of relay means 15. Generator 16 generates sampling pulses at a predetermined frequency which is equal to the control cycle.

MODE OF OPERATION A feed water flow rate F w is set into the feed water flow rate setter 2 when the boiler is started, and the output of setter 2 is applied to flow rate controller 8so that the feed water is supplied to boiler l at a predetermined flow rate. The output of feed water setter 2 is also applied to the first proportional arithmetic operation means 4 so that it is converted into a signal representing a basic fuel flow rate Ffb to be applied to first adder 5.

A temperature rise rate H for each control cycle AT is set into temperature rise setter 7, and the output of setter 7 is increased by an increment equal to H in response to each pulse received from sampling pulse generator 16 through closed relay means 15. The output signal is supplied to second adder 8.

In a first step just after starting, the temperature rises with the set feed water flow rate Fw and the basic fuel flow rate Ffl). After AT, representing a control cycle, a sampling pulse is applied to temperature rise controller 7 to set the latter to l X H automatically. The difference, Tw-Tw (0), between the temperature Tw at the outlet of the water-cooled furnace wall which is continuously detected by temperature detector 11, and the initial temperature Tw (O) of water at the outlet of the water-cooled furnace wall as set into the initial feed water temperature setter is derived from third adder 12 and is sampled by sampling means 13 and holding means M so that [1 X H- (tw( l )TW(0))] may be derived from adder 8. The output of adder 8 is supplied to second proportional arithmetic operation means 9, As a result, for the control cycle AT, the feed water is increased in temperature with the fuel flow rate (Ffb Ffc(l A similar operation occurs in each control cycle.

In the i-th step, that is, at a time iAT after starting, the temperature rise setter 7 is set to iH, and the difference Tw Tw(O) between the temperature Tw and the initial temperature Tw(O) is derived from third adder 12. This difference is converted into the form of Tw(z)- Tw(O) through sampling means 13 and holding means 14, and is fed back t5 second adder 8 as a negative signal to which is applied the signal M from temperature rise setter 7. The output signal iH {Tw(i) Tw(O)} from second adder is converted by second proportional arithmetic operation means 9 into the fuel flow rate correction signal Ffc(i) which is applied to first adder means 5 where it is added to the basic fuel flow rate signal Ffb so that the fuel flow rate signal Ff(i), is derived. In response to the fuel rate signal, Ff(i), fuel flow rate controller 6 controls the flow rate of fuel to be supplied to the starting burner of boiler l.

The mode of operation of the temperature rise rate circuit is illustrated in FIG. 3. From this figure, it will be seen that the temperature of the water at the outlet of the water-cooled furnace wall is increased at a predetermined temperature rise rate by the circuit just described and based upon the principle described. FIG. 3 is a graph illustrating the data obtained by the digital simulation of a supercritical forced once-through boiler of 600 MW with AT 10 minutes, H 367C, and Ew 5 percent. In FIG. 4, the broken line curve indicates the temperature rise when the fuel flow rate was kept constant, that is, the fuel flow rate was not controlled by the system of the invention.

The gas temperature control circuit also shown in FIG. 2, will now be described. The principle of operation is that when, and only when, the detected temperature of the gases at the outlet of the furnace is in excess of a predetermined value, the fuel flow rate is reduced irrespective of the fuel flow rate signal from the temperature rise rate circuit. That is, the gas temperature circuit which controls the fuel flow rate in response to the detected temperature is a means for protecting or safeguarding the operation of a plant.

The response of the gas temperature to the variation in fuel flow rate is quick, so that the gas temperature must be scanned and sampled in a cycle shorter than the sampling cycle used in the temperature rise rate circuit. For example, the sampling cycle in the gas temperature control circuit is 110 of that of the temperature rise rate circuit. The gas temperature circuit may be designed as a digital circuit, but it is preferable to design it as an analog circuit because it is used for monitoring and protecting the operation of a boiler plant.

In FIG. 2, the gas temperature circuit is illustrated under and right of the dash..dash chain line, and includes a gas temperature detecting means 21 which continuously detects the gas temperature at the outlet of the furnace and a limit setting means 22. The upper limit of the gas temperature at the outlet of the furnace of boiler 1 is set into the limit setting means or setter 22 which, in turn, generates an output signal represent ing this upper limit. The positive input of the temperature detecting means 21 is applied to a first input terminal of a fourth adder 23, and the negative input from limit setter 22 is applied to a second input terminal of this adder. An arithmetic unit 24 is connected to the output of fourth adder 23, and the output terminal of unit 24 is connected to the third input terminal of first adder 5 to apply a negative input thereto.

Arithmetic unit 24 has a backlash chracteristic so that when a negative input is applied thereto, it does now produce an output signal. When a positive input is applied thereto, unit 24 puts out an output signal corresponding to the positive input signal. A comparator 25 is connected to the output of unit 24, and its output is connected to the exciting terminal of relay means 15. When a positive input is applied to comparator 25, it generates a constant voltage signal for energizing relay means 15.

MODE OF OPERATION The gas temperature Tg and the upper limit TgL of the gas temperature set into the setting means 22 are compared by fourth adder means 23, and the difference signal is applied to arithmetic unit 24 which, as stated, has a backlash characteristic. When the gas temperature Tg is lower than the upper limit TgL, the output of adder 23, which is the input of arithmetic unit 24, is minus or negative so that arithmetic unit 24 does not provide any output signal. As a result, the fuel flow rate is controlled only by the temperature rise rate circuit.

When the gas temperature Tg exceeds the upper limit TgL, a positive signal is applied to arithmetic unit 24 so that the fuel flow rate correction Fg is determined, in response to the temperature difference, by unit 24 and applied as a negative input to first adder 5. As a result, the fuel flow rate is reduced accordingly by fuel flow rate controller 6 so that the gas temperature at the outlet of the furnace is decreased in proportion.

The output of arithmetic unit 24 is also applied to comparator 25 so that the latter provides a signal actuating relay 15, thereby short-circuiting the circuit which transmit the sampling pulses. As a result, the interference between the fuel flow rate correction signal from the gas temperature circuit and the fuel flow rate correction signal from the temperature rise rate circuit may be prevented. That is, when relay 15 is opened, the sampling pulse is skipped or interrupted so that sampling means 13 and temperature rise rate setter 7 are deactivated. Consequently, the fuel flow rate correction Ffc(i) derived from arithmetic operation unit 9 is maintained so that the fuel flow rate is controlled in response to the output signal of the gas temperature circuit.

Arithmetic operation unit 24 has a backlash characteristic in order to eliminate cycling and hunting. Therefore, when the fuel flow rate is decreased so that the gas temperature is reduced to less than (TgL ATg), where ATg is the backlash, the output signal of arithmetic unit 24 becomes 0. As a result, the output of comparator 25 also becomes 0 so that relay 15 is closed. Thus, the sampling pulses are applied to sampling means 13 and temperature rise rate setting means 7 so that the fuel flow rate is controlled in response to the temperature rise rate circuit in the manner described above.

FIG. 5 graphically illustrates the mode of operation of the gas temperature circuit, and it is believed that additional explanation is not necessary.

A control computer may be used for driving the feed water command and the fuel command based upon the above-described settings and arithmetic operations but, in accordance with the present invention, such a computer is not used. Instead, conventional control devices, such as potentiometers, integraters, and logic circuits using relays are employed, as will be described with reference to FIG. 6. In addition to the circuit components already described, FIG. 6 further includes means for automatically changing the temperature rise rate, means for permitting a holding control, means for automatically setting the initial temperature of the feed water, and means for automatically switching into the holding control when the deviation of the control variable from the setting point is in excess of a predetermined value.

Referring to FIG. 6, the automatic control system includes a pulse generator circuit (a), a change rate generator circuit (b), a setting point generating circuit (0), a controller circuit (d) and a circuit (e) for controlling the gas temperature at the outlet of the furnace. The pulse generator circuit (a) is shown at the upper right in FIG. 6 and includes a first flip-flop having a set terminal connected to a start button 32 and a reset terminal connected to a stop button 33. A first AND gate has a first input terminal connected to the output terminal of first flip-flop 31, and a second flip-flop 35 has a set terminal connected to the output terminal of first AND gate 34. A first delay circuit 36 has an input terminal connected to the output terminal of second flip-flop 35, and an output terminal connected to the reset terminal of flip-flop 35. In response to the input signal, first delay circuit 36 provides an output signal I, see. after an input is applied thereto.

A first NOT gate 37 has an input terminal connected to the output terminal of second flip-flop 35, and a second delaycircuit has an input terminal connected to the output terminal of first NOT gate 37 and an output terminal connected to the second input terminal of first AND gate 34. Second .delay circuit 38 delays-the input signal t see.

A third flip-flop 39 has a set terminal connected to a hold button-40 and a reset terminal connected to a reset button 41, and an OR gate 42 has a first input terminal connected to the output terminal ofthe third'flipfiop 39. A second NOTgate 43 has an input terminal connected to the output terminal of OR .gate 42, anda second gate 44 has a first input terminal connected to the output terminal of second NOT gate 43 and a second input terminal connected to the output terminal of second flip-flop 35.

A third NOT gate 45 has aninputterminal connected to the output terminal of a second abnormal condition detecting circuit 95 described hereinafter. A third AND gate 46 has its first input terminal connected to the output terminal of second flip-flop 35 and a second input terminal is connected to the output terminal of third NOT gate 45. It will be noted that first flip-flop 31 provides an output control signal R1, second ANDgate 44 provides an output signal R3 and third AND gate 46 provides an output signal R2.

The change rate generating circuit (b) is shown atthe left in FIG. 6, and includes a feed water temperature detector 51 which continuously detects the temperature Tw of water at the outlet of the water-cooled furnace wall of a boiler, which has not been shownin FIG. 6,.and provides a signal representing the detected temperature. Detector 51 corresponds to the detector 11 of FIG. 2. The change rate generating circuit also includes a first function generator 52 having the output of temperature detector 51 supplied thereto.

The setting point generating circuit as shown .at the lower left in FIG. .6, and includes a first setting point center 61 into which isset the final desired .temperature Twt( l) for starting the boiler, and which setter provides a signal representing the setting point. Thiscircuit also includes a second setting point setter 62 in which is set the setting point Twt(2) for full operationofthe once-through boiler, and this setter provides a signal representing this latter setting point. A changeover switch 63 is provided to select theoutput of either the first setter 61 or the second setter 62, and is connected to a first adder 64.

The output of adder 64 is supplied to an upper :and lower limit setter 65. When the absolute value of the output of first adder 64 is smaller than the output of first function generator 52 in change rate generating circuit (b), the outputof first adder 64 is derivedasthe output of the upper and lower limit setter 65. However, I

when the output of first adder 64 is greater than the input, the upper limitof the function generator is derived as the output whereas, when the input is smaller than the output of first function generator, the lower limit of function generator 52 is derived as the output.

Asecond adder has a first input terminal to which is supplied a positive inputprovided by the output'Tw .of temperature detector 51 in change rate generated circuit (b). A first relay switch 67 has an input contact connected to the output terminal of upper and lower limit setter 65, and is adapted to be closed only when the output pulse R3 of second AND gate in :pulse generating circuit (a) is applied thereto. A second relay switch 68 has a firstinput contact 68a connected to the output terminal of first relay switch 67, and asecond input contact 68b is connected to the output terminal of second adder 66. When the output pulse R1 of flipto flop 3l in pulse generating circuit (a) is applied torelay switch 68, first input contact '68ais connected to'output contact 68c. in the absence of pulse -R1, second input contact 68b is connected to'output contact 68c.

Anintegrater 69 has an input terminal connected :to the output terminal of second relay switch 68,.andhas1an output terminal connected to the secondinputlterminal of :each of the first and second adders 64 and 66 to apply negative inputs to the latter.

Thevcontroller circuit (d) as shown at the lower right in'FIG. 6 and includes a third adder 71 which has a first input'connected to the output terminal of integratorj69 so that its output Ts is applied to third adder'7l as a positive input. The second input of adder 71 is connectedxtotheoutput of temperature detector 51 so that the output Tw of detector 51 is applied as a negative input to third adder 71. The output of third adderis'ap- :relay switch 75 is connected between the output of .servo amplifier '73 and motor 76, and is closed when,

and only when, the output pulse "R2 of third AND gate 46 is applied thereto.

Motor 76 is energized only when one or the-other of the relay -.switches74l or 75 is closed, andactuates apotentiometer77 which provides an output toithe second input terminal of fourth adder 72 as a negative input thereto. A proportional arithmetic operation unit 78 hasan input terminal connected to the output of potentiometer 77 and corresponds to the means 9 shown :in FIG.'.2. A fifth adder 79 has a first input terminal to which there is applied, as a positive input, the output .of unit 78, and the basic fuel flow rate signal Fjb from the feed water flow rate setting means, which has not been shown in FIG. 6, is applied to the second input terminal of .adder 79. The output Ffli) issupplied .to a fuel flow rate control means which has not'beenshown in FIG. 6. Adder 79 corresponds to the means 6 shown in FIG. 2. g

A first abnormal condition detecting circuit 80 has an input terminal connected to the output terminal of third adder7l and an output terminal-connected to the second input terminal of OR gate 42 in circuit(a). The first abnormal condition detecting circuit 80 provides an output signal only when the aboslute value of its input is in excess of a predetermined value The circuit for the temperature of gases at the outlet of the furnace is shown at the upper left in FIG-6., and includes a gas temperature detector 91 which continu ously detects the temperature Tg of gases at the outlet of the furnace of the boiler and provides a-signal representing the detected temperature. Detector 91 corresponds to the means 21 shown in FIG. 2. A limit value setter 92 provides an output signal, representing the upper limit, TgLof the temperature of gases at the-outlet of the furnace, and corresponds to the means 22 shown in FIG. 2. Asixth adder 93 has the output of detector '91 applied as a positive input to its first input terminal, and the outputof limit value setter 92 is applied to the second input terminal as a negative input. Adder 93 corresponds to the means 23 shown in FIG. 2.

A second function generator 94 has an input terminal connected to the output terminal of sixth adder 93 and an output terminal connected to the third input terminal of fifth adder 79 to apply a negative input thereto. Function generator 94 has a backlash characteristic and provides no output signal when a negative input is applied thereto, but does apply an output signal responsive to a positive input. Generator 94 coresponds to the means 24 shown in FIG. 2.

The second abnormal condition detecting circuit 95 has an input terminal connected to the output terminal of sixth adder 93, and provides an output signal when, and only when, the input is in excess of a predetermined value 6,.

MODE OF OPERATION Pulse generating circuit (a) generates three pulse signals in order to control the setting point generating circuit (c) and the controller circuit (d). The first pulse is the starting pulse R1, which is the output of first flipflop 31 and serves for starting and stopping the temperature rise control, this pulse being produced response to operation of the start push button 32 and being terminated responsive to operation of the stop push button 33. Pulse generating circuit (a) produces control pulses R2, which represent the output of third AND gate 46, and are used for intermittent control. Finally, the circuit produces holding pulses R3, which represent the output of second AND gate 44 for holding control.

The holding control is effected when the output is 0.

Change rate generating circuit (b) generates the temperature rise rate of water at the output of the outlet of the water-cooled furnace wall as a function of the temperature at the outlet of the water-cooled furnace wall. The output of this circuit, which is a desired temperature rise rate, is applied to the setting point setting generating circuit (c).

This latter circuit generates the setting point Ts in one step based upon the final setting point of the water at the outlet of te water-cooled furnace, all set into the setting point setters 61 or 62, and the desired temperature rise rate from the change rate generating circuit (b). The setting point generating circuit functions in three modes, as follows.

in a follow-up mode, the starting pulse R1 is not generated, and the output Ts of the setting point generating circuit is always following up the detected temperature Tw of the water at the outlet of the water-cooled furnace wall.

In the temperature rise mode, not only the starting pulses R1, but also the holding pulses R3, which are equal in phase and timing to the control pulses R2, are generated. However, this is not the holding mode. The setting point, in one step, of the temperature of water at the outlet of the water-cooled furnace wall, changes in response to the desired temperature rise rate derived from the change rate generating circuit (b), and the output Ts is applied to the controller circuit (d).

In the holding mode, the starting pulse R1 is generated but the holding pules R3 are not generated. The output Ts of the setting point generating circuit is maintained constant.

Turning now to the controller circuit (b), the deviation of the temperature Tw of water at the outlet of the water-cooled furnace wall from the setting point Ts is derived from third adder 71 and applied to the controller circuit. When the starting pulse R1 is generated, po-

tentiometer 77 is driven by motor 76 when, and only when, a control pulse R2 is generated, so that the output of the potentiometer equals the output (Ts Tw) of third adder means 71 as third relay switch 74 is opened. When a control pulse R2 is not generated, the existing condition is maintained. The intermittent signal for correcting the fuel flow rate is generated, and the output in proportion to this signal is applied through fifth adder 79 to the fuel flow rate controller which has not been shown in FIG. 6.

When the deviation (Ts Tw) is too great, the fixed value mode is required, so that no pulse R3 appears in pulse generating circuit (a) and the existing condition is maintained.

In the circuit (e) for the temperature of gases at the outlet of the furnace, when the gas temperature Tg at such outlet is in excess of the limits TgL, the fuel flow rate correction Fg, which is proportional to the temperature difference, is determined by second function generator 94 and applied to fifth adder 79 as a negative input. As a result, the fuel flow rate is reduced by the fuel flow rate controller so that the gas temperature at the outlet of the furnace is decreased accordingly.

When the temperature difference (Tg TgL) is too great, pulse generating circuit (a) will not produce pulses R2 ad R3, so that the output of the proportional arithmetic operation unit 78 remains unchanged. As a result, the fuel flow rate Ff is controlled only in response to the circuit for the temperature of gases at the outlet of the furnace.

MODE OF OPERATlON OF THE OVERALL SYSTEM In the initial stage, when the control system is coupled to a power source, pulse generating circuit (a) will not generate any pulses and, in change rate generating circuit (b), first function generator 52 provides, as an output, the desired temperature rise rate signal which is the function of the detected temperature of water at the outlet of the water-cooled furnace wall and is set into setting point generating circuit (0). ln setting point generating circuit (0), the second input contact 68b of second relay switch 68 is connected to contact 68c because no starting pulse R1 is applied, so that the setting point generating circuit is set into the follow-up mode in which the output signal is always equal to the detected temperature Tw at the outlet of the watercooled furnace wall.

In the controller circuit (d), the output of third adder means 71 is 0, because the output Ts of the setting point generating circuit equals the detected temperature, and third relay switch 74 is closed because no starting pulse R1 is generated. Thus, the potentiometer 77 which is driven by motor 76 always follows up the output of third adder means 71, the output or the fuel flow rate correction signal Ffc of the controller circuit being 0. In the circuit (2), the output (Tg TgL) of sixth adder 93 is negative because the detected gas temperature Tg is less than the upper limit TgL, so that the output Fg of second function generator 94 is 0.

For temperature rise control, start button 72 is depressed and start pulse R1 is generated from first flipflop 31 and applied to setting point generating circuit (c) and controller circuit (d). The start pulse R1 is also applied to the first input terminal of first AND gate 34.

At this point in time, no output signal is derived from second flip-flop 35, so that the output signal is derived from first NOT gate 37 and applied to the second input terminal of first AND gate 34. As a result, an output signal is derived from AND gate 34 and applied to the set terminal of second flip-flop 35. The output signal of flip-flop 35 is applied to the reset terminal thereof t, see. later, so that flip-flop 35 does not provide any output signal. Thus, the output signal is derived from NOT circuit 37 and is applied to the second input terminal of first AND gate 34, so that the output signal of this AND gate is applied to the set terminal of flip-flop 35, which then provides an output signal.

The output of second-flip-flop 35 is applied to the first input terminal of third AND gate 46 as an intermittent pulse which appears for t, secs. and disappears for sec. As long as there is no output signal from second abnormal condition detecting circuit 95, third NOT 45 provides a signal to the second input terminal of third AND gate 46, so that intermittent pulses, which are equal in phase and timing to the output of second flipflop 35 are derived at the output terminal of third AND gate 46 and applied as control pulses R2 to controller circuit (d).

The output of second flip-flop 35 is also applied to the second input terminal of second AND gate 44. Thus, if the hold condition is not maintained, that is, when the output of OR gate 42 is 0, the output signal is derived from second NOT gate 43 and applied to the first input terminal of second AND gate 44. Holding pulses R3, whose phase and timing are equal to those of the output of second flip-flop 35, are derived from AND gate 44 and applied to setting point generating circuit (c). If the control system is in the hold mode, pulses R3 are always 0.

In the change rate generating circuit, during the initial stage, the desired temperature rise rate signal in response to the detected temperature at the outlet of the water-cooled furnace wall is transmitted to setting point generating circuit In this latter circuit, and in response to starting pulse R1 from the pulse generating circuit, second relay switch 68 engages first input contact 68a, so that the circuit is switched to the temperature rise mode from the followup mode. In a substantial part of the temperature rise control stage, the final setting temperature Twt (l) is sufficiently higher than the setting temperature Ts in each step, with the difference between them being the output of first adder 64, is higher than the output of first function generator 62. As a result, the output of the upper and lower limit setter 65 equals the output of first function generator 52. For the time sec. during which pulse R3 is transmitted, the desired temperature value in each step, that is, the output Ts, is increased with a constant increase rate, the temperature rise mode, with a desired change rate from the change rate generating circuit (b), and the output Ts is maintained at a constant value, namely, the holding mode, for a time interval t sec. during which there is no pulse R3. The temperature rise mode and the fixed value mode are carried out alternately, so that the desired temperature value, or the output Ts of the setting point generating circuit, is increased stepwise.

Turning now to the controller circuit (d), in response to starting pulse R1, third relay switch 74 is opened so that interconnection between servo amplifier 73 and motor 76 is either maintained or is broken depending upon whether fourth release switch is closed or opened.

Third adder 71 derives the difference between the detected temperature Tw of the water at the outlet of the water-cooled furnace wall and the output Ts of the setting point generating circuit, that is, the setting point in each step. The difference (Ts Tw) is applied to potentiometer 77 driven by motor 76. Potentiometer 77 follows up in a manner such that it provides an output signal equal to the output (Ts Tw) of third adder means 71 for a time interval during which control pulse R2 is applied, and then holds the existing condition for a time interval sec. during which there is no control pulse R2. The above operations are carried out alternately.

The output signal derived from potentiometer 77 is multiplied by proportional arithmetic operation units 78 to provide the signal Ffc. This signal is added to the basic fuel flow rate signal Ffb as a fuel flow rate correction signal in fifth adder 79, to be applied to the fuel flow rate controller.

In circuit (e), the gas temperature Tg at the outlet of the furnace, as detected by detector 91, and the upper limit of the gas temperature TgL, set by limit setter 92 are compared by sixth adder 93, and the difference signal is applied to second function generator 94. When the detected gas temperature Tg is less than the upper limit TgL, the output of sixth adder 93, which is the input signal to second function generator 94 is negative, so that no signal is derived from function generator 94. As a result, the fuel flow rate is controlled in response to the correction signal Ffc.

When the gas temperature Tg becomes higher than the limit TgL during the temperature rise control stage, a positive input signal is applied to second function generator 94 so that the fuel flow rate correction signal Fg is determined in response to the temperature difference, and is applied as a negative input to fifth adder 79. Consequently, the fuel flow rate if reduced by the fuel flow rate controller. Pulse generating circuit (a) generates 0 holding pulses R3, so that the output Ts of setting point generating circuit (0) becomes constant and the temperature rise is interrupted. Additionally, control pulses R2 become 0, so that the output Ffc of proportional arithmetic unit 78, that is, the fuel flow rate correction signal required for temperature rise control, remains unchanged so that the fuel flow rate Ff is controlled in response to the gas temperature Tg at the outlet of the furnace and the gas temperature Tg is reduced.

The control may be switched to the holding control mode in which the output Ts of circuit (c) is maintained constant. In the holding control mode, there may be effected an operation in response to a request from the operator. Thus, when the operator depresses hold button 40, third flip-flop 39 is set so that an input signal is applied to OR gate 42. Additionally, a selfholding control operation may be effected. When the difference between the output Ts of circuit (0) and the detected temperature Tw of water at the outlet of the furnace wall is in excess of a predetermined deviation e., or when the difference between the detected gas temperature Tg and the upper limit TgL is in excess of a predetermined deviation 6 so that an abnormal temperaturerise control is detected by the first or second abnormal condition detector or 95, respectively, an

input signal is applied to the second or third, respectively, input terminal of OR gate 42.

As a result, an input is applied to second NOT gate 48 from OR gate 42, but NOT gate 48 will not provide any output signal so that the output of second AND gate 44 constitutes a hold signal as no pulse R3 is present. Consequently, first relay switch 67 is opened so that circuit is switched to the holding mode in which its output is maintained constant. Control pulse R2 is applied also to controller circuit (d) so that a holding control is carried out.

When the holding control has been set by the operator, it may be released by the operator depressing reset button 41 to release the holding control.

With a self-holding control mode, when the difference between Ts and Tw is brought within a predetermined deviation 6,, or when the difference between Tg and TgL is brought within the predetermined deviation 6 the holding control is automatically released.

In transition to a normal operation, setting point generating circuit (0), has its output Ts alternately increased for a time interval 1 sec. in response to the change rate signal from circuit (b) and then maintained at the existing level for the next time interval t sec., until the desired setting temperature Twt( l) is attained when the boiler is operated by the starting system.

When switching from the starting operation to the normal or full operation, changeover switch 68 is automatically or manually switched, so that the setting point is changed to the desired temperature Twt(2) in the normal or full operation and the temperature rise control is continued.

FIG. 7 graphically illustrates data obtained by experiments conducted at a supercritical test plant, having a forced once-through boiler with a capacity of 2 tons per hour steam flow and having a control computer.

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be uncircuit and to said pulse generating circuit and operable responsive to start and control pulses, and to deviation of said actual water temperature from said setting water temperature to provide a fuel flow rate correction signal in a direction to reduce such deviation; said control system increasing the water temperature at the outlet of said water-cooled furnace wall stepwise responsive to a start pulse and to said control pulses at a rate change controlled by said rate change generating circuit.

2. A control system, as claimed in claim 1, further including second detector means measuring the gas temperature at the outlet of said furnace; and a gas temperature control circuit connected to said second detector means, to said pulse generating circuit and to said controller circuit and operable to reduce the fuel flow rate correction and to interrupt said control pulses when the gas temperature at the outlet of the furnace is in excess of a predetermined level.

3. A control system, as claimed in claim 2, including first abnormal condition detector means connected to said pulse generating circuit and to said controller circuit and operable to interrupt said control pulses when said actual water temperature is abnormal or in excess of a prdetermined level; and second abnormal condition detector means connected to said gas temperature control circuit and to said pulse generating circuit and operable to interrupt said control pulses when the gas temperature at the outlet of said furnace is abnormal or in excess of a predetermined level.

4. A control system, as claimed in claim 1, in which said setting point generating circuit is operable, in the absence of a start pulse, to generate a setting water temperature equal to said actual water temperature at the outlet of said water-cooled furnace wall.

5. A control system, as claimed in claim 4, in which said setting point generating circuit is operable, responsive to a start pulse but in the absence of holding pulses demoed that the invention may be embodied Othep to maintain said setting water temperature ataconstant wise without departing from such principles.

What is claimed is:

l. A control system for the starting phase of a oncethrough boiler comprising, in combination, a pulse generating circuit generating a start pulse, control pulses having a predetermined duration and predetermined intervals, and holding pulses having a phase and timing equal to those of said control pulses; first detector means measuring the actual water temperature at the outlet of a water-cooled wall of the furnace of said boiler; a rate change generating circuit connected to said first detector means and generating a desired temperature rise rate automatically in response to a controlled variable; a setting point generating circuit connected to said rate change generating circuit and to said pulse generating circuit and operable to set a setting water temperature at the outlet of said watercooled wall responsive to temperature rise rate signals from said rate change generating circuit; and a controller circuit connected to said setting point generating level.

6. A control system, as claimed in claim 5, in which said setting point generating circuit is operable responsive to start and holding pulses to vary said setting water temperature during consecutive predetermined time intervals responsive to a desired temperature rise rate signal from said rate change generating circuit.

7. A control system, as claimed in claim 6, including manually operable means selectively operable to condition said setting point generating circuit to maintain said setting water temperature at a constant level.

8. A control system, as claimed in claim 2, including a first setting point setter operable to set a setting water temperature during the starting phase of said boiler and a second setting point setter operable to set a setting water temperature during normal operation of said boiler; and switch means selectively operable to connect either of said first and second setting point setters to said setting point generating circuit. 

1. A control system for the starting phase of a once-through boiler comprising, in combination, a pulse generating circuit generating a start pulse, control pulses having a predetermined duration and predetermined intervals, and holding pulses having a phase and timing equal to those of said control pulses; first detector means measuring the actual water temperature at the outlet of a water-cooled wall of the furnace of said boiler; a rate change generating circuit connected to said first detector means and generating a desired temperature rise rate automatically in response to a controlled variable; a setting point generating circuit connected to said rate change generating circuit and to said pulse generating circuit and operable to set a setting water temperature at the outlet of said water-cooled wall responsive to temperature rise rate signals from said rate change generating circuit; and a controller circuit connected to said setting point generating circuit and to said pulse generating circuit and operable responsive to start and control pulses, and to deviation of said actual water temperature from said setting water temperature to provide a fuel flow rate correction signal in a direction to reduce such deviation; said control system increasing the water temperature at the outlet of said water-cooled furnace wall stepwise responsive to a start pulse and to said control pulses at a rate change controlled by said rate change generating circuit.
 2. A control system, as claimed in claim 1, further including second detector means measuring the gas temperature at the outlet of said furnace; and a gas temperature control circuit connected to said second detector means, to said pulse generating circuit and to said controller circuit and operable to reduce the fuel flow rate correction and to interrupt said control pulses when the gas temperature at the outlet of the furnace is in excess of a predetermined level.
 3. A control system, as claimed in claim 2, including first abnormal condition detector means connected to said pulse generating circuit and to said controller circuit and operable to interrupt said control pulses when said actual water temperature is abnormal or in excess of a prdetermined level; and second abnormal condition detector means connected to said gas temperature control circuit and to said pulse generating circuit and operable to interrupt said control pulses when the gas temperature at the outlet of said furnace is abnormal or in excess of a predetermined level.
 4. A control system, as claimed in claim 1, in which said setting point generating circuit is operable, in the absence of a start pulse, to generate a setting water temperature equal to said actual water temperature at the outlet of said water-cooled furnace wall.
 5. A control system, as claimed in claim 4, in which said setting point generating circuit is operable, responsive to a start pulse but in the absence of holding pulses to maintain said setting water temperature at a constant level.
 6. A control system, as claimed in claim 5, in which said setting point generating circuit is operable responsive to start and holding pulses to vary said setting water temperature during consecutive predetermined time intervals responsive to a desired temperature rise rate signal from said rate change generating circuit.
 7. A control system, as claimed in claim 6, including manually operable means selectively operable to condition said setting point generating circuit to maintain said setting water temperature at a constant level.
 8. A control system, as claimed in claim 2, including a first setting point setter operable to set a setting water temperature during the starting phase of said boiler and a second setting point setter operable to set a setting water temperature during normal operation of said boiler; and switch means selectively operable to connect either of said first and second setting point setters to said setting point generating circuit. 